1. Field of the Invention
This invention relates to a watermattress apparatus having a water bag which contains a liquid.
2. Description of the Related Art
There are various types of mattress apparatuses, which include a watermattress apparatus. The watermattress apparatus has a water bag which contains a liquid. The liquid flows in accordance with the irregularity of the body of a user lying thereon. Thus, the watermattress apparatus is free from a reactive force, which is created in a mattress with a spring unit when the user lies thereon, and corresponds to the amount of compression of the spring unit. Therefore, a heavy portion of the body, such as the hip, is prevented from being strongly stressed.
The watermattress apparatus constructed as above is very advantageous when used as a bed for e.g. a patient or an old man who must lie thereon for a long time, since the apparatus can prevent a bedsore. In light of this, watermattress apparatuses have been widely used.
The watermattress apparatus generally has a case which includes a receiving portion receiving the above-described water bag. This case is covered with an envelope made of cloth.
The liquid contained in the water bag is incompressible though it deforms the water bag during flowing. Thus, the watermattress apparatus receiving the water bag does not have such comfortable cushion properties as a spring mattress has. In other words, the watermattress does not provide comfortable elastic compression when the weight of the user has been exerted thereon.
Further, the liquid contained in the water bag becomes very cold in winter. To prevent this, a heater is provided in the case receiving the water bag to heat the liquid to a temperature comfortable to the user.
It is said that the temperature which enables the user to have a comfortable sleep is about 33.degree..+-.1.degree. C., and that the user can have a good sleep when the temperature of the legs of the user is higher by 3.degree. C. than the other portions of the body. If, however, the user is oversensitive to cold, the temperature of the legs is generally lower by 3.degree. C. than the other portions of the body, a comfortable sleep cannot be obtained unless the legs are heated at a higher temperature than the other portions.
The above-described water bag is formed of a flexible and waterproof sheet member which is made of rubber or vinyl. If the water bag is stung by a knife or the like, or receives a great shock, or deteriorates with age, the liquid contained therein may leak from the bag to the receiving portion of the case.
To prevent the liquid from leaking from the case, a waterproof bottom sheet is provided in the receiving portion. This bottom sheet, however, cannot completely prevent penetration of the leaked liquid into the envelope and then to the outside thereof.
In order to prevent the penetration of the leaked liquid into the envelope, it is considered to cover, with an upper sheet, the upper side of the receiving portion, to which the receiving portion opens.
However, where the upper sheet is provided between the case and the envelope, the sweat of the user body lying on the envelope or moisture in the atmosphere can easily gather on the upper sheet. Since the upper sheet is in contact with the water bag and hence has a low temperature, the moisture which contacts the upper sheet condenses into a dew. It is possible that a great amount of dews gathers on the upper sheet and penetrates into the envelope which contacts the upper sheet, with the result that spots appear on the envelope. Moreover, the envelope is liable to have a moisture, which may give the user a bad feeling, may cause propagation of bacteria, or may emit off-flavor.